Charge transfer devices such as CCDs have gained popularity in recent years for performing a wide variety of electronic functions including signal processing, signal delay and image sensing.
A CCD imager, for example, includes an array of charge collecting regions formed in a semiconductor substrate which is responsive to an incident radiation image for developing image-representative charge packets. The output of a CCD imager, as well as a CCD signal processor or delay line, includes a line register which receives the charge packets and serially transfers them to a charge sensing stage. The charge sensing stage includes a periodically reset floating element for developing a voltage signal in response to sensing of the serially supplied charge packets. Typically, an on-chip FET amplifier (electrometer) connected to the floating element provides the imager output signal. The FET amplifier contributes a noise component to the imager output signal commonly called "1/f" noise. The process of periodically resetting the floating element undesirably results in a "reset noise" component appearing in the imager output signal. Reset noise results from variations in potential left upon the floating element from one reset interval to the next. At the lower-video frequencies of the imager output signal 1/f noise predominates. At the upper-video frequencies of the imager output signal, reset noise predominates. Typically the reset noise is about 8 db larger than the 1/f noise.
L. N. Davy, in his U.S. Pat. No. 4,330,753 issued May 18, 1982 and entitled Method and Apparatus for Recovering a Signal From a Charge Transfer Device, describes a relatively low noise method for signal recovery from a charge transfer device output stage which includes a periodically reset FET coupled to a floating element. The FET output signal is passed through a bandpass filter (BPF) to separate out the double-sideband amplitude-modulated (DSB-AM) component centered about the sixth multiple of the clocking frequency. This DSB-AM signal is heterodyned using a synchronous detector type of demodulator to provide a baseband spectrum signal. The Davy method is effective for suppressing the 1/f noise component since the 1/f noise is principally confined to the lower frequency spectrum of the device output, which is suppressed by the BPF. Additionally, the relative duty factors of the clock signals and the device output signal are adjusted by Davy so that the amplitude of the clock signal components appearing about the sixth harmonic of the output signal are zero. This reduces clock feedthrough. It appears that although reset noise is not specifically identified by Davy, it may also be reduced by the above-noted adjustment of signals, however the information signal component of the output signal is relatively small at the sixth harmonic and therefore may have a poor signal-to-noise characteristic.
U.S. Pat. No. 4,556,851 issued Dec. 3, 1985 to Peter A. Levine entitled Reduction of Noise in Signal From Charge Transfer Devices and assigned, like the present application, to RCA Corporation, describes a charge sensing stage including a sample-and-hold circuit used as a detector. The sample-and-hold circuit operates in response to a sampling signal having a pulse repetition rate at the fundamental (rather than the sixth multiple) of the clock signal frequency for detecting the fundamental component of the output signal. An RC high-pass filter is used to differentiate the device output signal before its detection. The corner frequency of the RC high-pass filter is chosen to suppress the 1/f noise. Reset noise suppression is achieved by applying reset pulses to the floating element at times preceeding admission of charge packets under the floating element by intervals each substantially as long as the reciprocal of the corner frequency in radiants per unit time of the RC filter. Although this technique satisfactorily reduces the 1/f and reset noise, depending upon actual circuit construction techniques and layout, under varying temperature conditions the phase of the sampling signal applied to the detector may shift and give rise to an objectionable output signal shading component.
It is desirable to provide a charge sensing stage for a charge transfer device which reduces both 1/f and reset noise and is relatively insensitive to temperature variations.